Autism Spectrum Disorders (ASDs) are currently estimated to impact 1-2.6% of children world-wide, representing a steep rise in ASD prevalence with annual costs to the United States alone calculated at $126 billion (2012). Because known therapies are less effective with increasing age of diagnosis, addressing outstanding questions about ASD etiology is a research priority. In the more common mammalian models however, embryonic stages are inaccessible therefore embryogenesis presents a major gap in our understanding of ASD etiology. To address this gap, we propose to generate zebrafish ASD models to focus explicitly on functional consequences in embryos of mutations known to cause ASD. Rather than investigate social behaviors typically used to define ASD, we focus on internal phenotypes (endophenotypes) of neuroanatomy and physiology. Following this strategy, our preliminary data from morpholino knockdown experiments demonstrate common phenotypes when either of two distinct ASD-linked genes, SHANK3 and SYNGAP1, is knocked down in zebrafish. Common phenotypes include developmental delay and seizure-like behaviors. These seizure-like behaviors are likely explained by dramatic reductions in the numbers of inhibitory GABAergic neurons in both morphant models. Developmental delay, seizures, and reduced markers of GABAergic signaling are also characteristic of individuals with ASD. To follow up on these preliminary studies we propose to generate stable gene knock-outs of zebrafish shank3 and syngap1. By creating and analyzing stable mutant lines with respect to the development of GABAergic brain circuits, our goal is to determine the developmental mechanisms that underlie GABA deficits. In the long-term, these zebrafish ASD models can also serve as the basis for the discovery of therapeutic targets and environmental risk factors.